Oxygen transport membranes function to separate oxygen from air or other oxygen containing gases by transporting oxygen ions through a material that is capable of conducting oxygen ions and electrons at elevated temperatures. When a partial pressure difference of oxygen is applied on opposite sides of such a membrane, oxygen ions will ionize on one surface of the membrane and emerge on the opposite side of the membrane and recombine into elemental oxygen. The free electrons resulting from the combination will be transported back through the membrane to ionize the oxygen. The partial pressure difference can be produced by providing the oxygen containing feed to the membrane at a positive pressure or by combusting a fuel or other combustible substance in the presence of the separated oxygen on the opposite side of the membrane or a combination of the two methods. It is to be noted that the combustion will produce heat that is used to raise the temperature of the membrane to an operational temperature at which the oxygen ion transport can occur and also, to supply heat to an industrial process that requires heating. Moreover, whether or not heat is required for a process, the combustion itself can produce products such as synthesis gases by means of partial oxidation of a fuel or other combustible substance occasioned as a result of the combustion.
Oxygen transport membranes can utilize a single phase mixed conducting material such as a perovskite to conduct the electrons and transport the oxygen ions. While perovskite materials can exhibit a high oxygen flux, such materials tend to be very fragile under operational conditions involved where a fuel or other combustible substance is used to produce the partial pressure difference. This is because the perovskite will have a variable stoichiometry with respect to oxygen. In air, it will have one value and in the presence of a fuel that is undergoing combustion it will have another value. The end result is that at the fuel side, the material will tend to expand relative to the air side and a dense layer will therefore, tend to fracture. In order to overcome this problem, a mixture of materials can be used in which a primarily ionic conductor is provided to conduct the oxygen ions and a primarily electronic conductor is used to conduct the electrons. The primarily ionic conductor can be a fluorite such as a stabilized zirconia and the primarily electronic conductor can be a perovskite. Where the primarily ionic conductor is a fluorite, this chemical expansion is less problematical.
Typically, oxygen transport membranes are composite, also known as supported thick film, structures that include a dense separation layer composed of the two phases of materials, a porous fuel oxidation layer located between the dense separation layer and a porous support layer and a porous surface activation layer located opposite to the porous fuel oxidation layer and on the other side of the dense separation layer. All of these layers are supported on a porous support, or porous supporting substrate. The dense separation layer is where the oxygen ion transport principally occurs. Although defects in the dense separation layer can occur that enable the passage of gas through such layer, it is intended to be gas tight and therefore, not porous. Both the porous surface activation layer and the porous fuel oxidation layers are “active”, that is, they are formed from materials that permit the transport of oxygen ions and the conduction of electrons. Since the resistance to oxygen ion transport is dependent on the thickness of the membrane, the dense separation layer is made as thin as possible and therefore must be supported in any case. The porous fuel oxidation layer enhances the rate of fuel oxidation by providing a high surface area where fuel can react with oxygen or oxygen ions. The oxygen ions diffuse through the mixed conducting matrix of this porous layer towards the porous support and react with the fuel that diffuses inward from the porous support into the porous fuel oxidation layer. The porous surface activation layer enhances the rate of oxygen incorporation by enhancing the surface area of the dense separation layer while providing a path for the resulting oxygen ions to diffuse through the mixed conducting oxide phase to the dense separation layer and for oxygen molecules to diffuse through the open pore space to the dense separation layer. The surface activation layer therefore, reduces the loss of driving force in the oxygen incorporation process and thereby increases the achievable oxygen flux. Preferably, the porous fuel oxidation layer and the porous surface exchange layer are formed from the same electronic and ionic phases as the dense separation layer to provide a close thermal expansion match between the layers.
U.S. Pat. No. 7,556,676 describes a composite oxygen ion transport membrane. In order to form a dense, gas impermeable dual phase membrane layer from these materials the membrane needs to contain vanadium, and be sintered in a furnace atmosphere containing a mixture of hydrogen and nitrogen. From a cost perspective for high volume manufacturing it would be preferable to sinter in air. From an environmental viewpoint it would be beneficial to eliminate vanadium. The materials of both the porous intermediate fuel oxidation layer and the porous air side surface exchange layers described in this patent have shown a tendency to densify during prolonged usage at high temperatures. Densification of these layers results in degradation of oxygen flux through the membrane.
As will be discussed the present invention provides a method of manufacturing a composite oxygen ion transport membrane and the membrane structure resulting from such manufacture that among other advantages incorporates materials that enable fabrication to be accomplished in a more cost effective manner than in the prior art and also, will be more durable than prior art membranes.